This invention relates to the detection of nuclear radiation and to the determination of nuclear radiation dosages.
So-called thermoluminescent materials are known. A particularly well-known thermoluminescent material is lithium fluoride. Also, as described in South African patent application No. 86/1988, industrial diamonds are also known to have the characteristics of thermoluminescent materials. Another known thermoluminescent material is cubic boron nitride (CBN).
When a crystalline thermoluminescent material is subjected to nuclear radiation (such as radiation by X-rays, alpha particles, neutrons, protons, gamma rays or electrons) while at an appropriately low temperature, free electrons or holes become trapped at lattice imperfections in the crystal and can remain so trapped for a considerable period of time if their temperature is maintained at a sufficiently low value. However, when the temperature is increased, the electrons or holes return to stable energy stages, often with the emission of light.
This phenomenon has been used to determine nuclear radiation dosages in medical physics. For instance, a person who is subjected to nuclear radiation, such as X-rays, is fitted with a thermoluminescent crystal in which the above-described change in electron energy level takes place during exposure to the radiation. The crystal is then removed from the person of the wearer and is subjected to testing to obtain a measure of the dosage of radiation to which the crystal and hence the person has been subjected. The conventional testing technique involves placing the irradiated crystal on a support surface which is heated from below using a resistance heater. The light emitted by the crystal upon being heated is collected by a photomultiplier (PM) tube. The amount of light which is collected by the PM tube is then used to give a determination of the original nuclear radiation dosage in terms of a known relationship between light collected and radiation dosage.
While the principles of the technique are sound, great difficulty has been encountered in getting repeatability of results. The main cause for this variation in results is the requirement that the crystal be extremely accurately and uniformly positioned on its support. Failure to locate crystals in exactly the same way each time results in non-uniform results because of different heat intensities applied to the crystal and to non-uniform light collection by the PM tube. Furthermore, the crystal is heated from one side only and this contributes to non-uniformity during the heating process. Also, much of the light which the crystal emits is scattered and is not collected at all by the PM tube.
Other problems are that the emitted light, especially in the case of a thermoluminescent diamond, is of low intensity which again means that the diamond must be extremely accurately positioned in relation to the PM tube. Also, the conventionally used resistance heaters may take as long as 20 to 30 seconds to heat up a diamond to the required temperature.
It is believed that thermoluminescent diamonds could be used to good effect in medical physics to determine radiation dosage. The main reason is that diamond, being carbon, has a composition corresponding closely to that of the human body. This means that the radiation absorbed by a diamond will approximate to that absorbed by the human body and hence that diamond will give a reliable indication of dosage to which a human body has been exposed.
The present invention seeks to provide an alternative method and means for determining nuclear radiation dosages using thermoluminescent crystals.